Quinto first appeared on television in the short-lived television series The Others, and appeared as a guest star on shows including CSI, Touched by an Angel, Charmed, Six Feet Under, Lizzie McGuire, and L.A. Dragnet. In 2003, during the theatrical run of Endgame by Samuel Beckett directed by Kristina Lloyd[10] at the Odyssey Theatre in Los Angeles playing the role of Clov,[11] he landed a recurring role as computer expert Adam Kaufman on the Fox series 24; Quinto appeared in 23 episodes of the third season.

Quinto at the premiere party of Heroes' third season in 2008

In 2006, Quinto played the role of Sasan: the haughty, gay Iranian-American best friend of Tori Spelling on her VH1 series So NoTORIous. Later that year, he joined the cast of Heroes as Gabriel Gray, better known as the serial killer Sylar.[12] He worked on the series until its cancellation in 2010 after four seasons.

His casting as a young Spock in the J. J. Abrams-directed reboot of the Star Trek film franchise was officially announced at the 2007 Comic-Con.[13][14] Speaking alongside Leonard Nimoy at a press conference to promote the new Star Trek film, Quinto revealed that Nimoy had been given casting approval over who would play the role of the young Spock. "For me Leonard's involvement was only liberating, frankly,"[15] says Quinto. "I knew that he had approval over the actor that would play young Spock, so when I got the role I knew from the beginning it was with his blessing."

In a September 2008 interview, Abrams said of Quinto's performance as Spock: "Zachary brought a gravity and an incredible sense of humor, which is a wonderful combination because Spock's character is deceivingly complicated. The revelation for me watching the movie, when I finally got to watch the whole thing after working on sequences, was that he is extraordinary. He was doing things I didn't even realize while we were shooting – these amazing things to track his story."[16] Quinto also made references to Star Trek's historical record for diversity and inclusiveness in its casting and storylines, and said that he hoped the looming election of Barack Obama would build that dynamic towards the film's May 2009 release date.

Following Star Trek, he appeared in the comedy shortBoutonniere (2009). It "...was a movie written and directed by my former landlady and friend, [actress Coley Sohn]. She called up and said, 'Would you do me a favor and be in my short film?'"[17]

Quinto has joined with Corey Moosa and Neal Dodson to form Before the Door Pictures.[18] The company is working on projects in film, television, new media, and the graphic novel arena. It announced a three-book publishing deal with comic book publisher Archaia at the 2009 San Diego Comic-Con. The first project from the partnership is expected to be a 100-page graphic novel called Mr. Murder is Dead, created by writer Victor Quinaz. It will be followed by the comic book series LUCID: A Matthew Dee Adventure written by writer/actor Michael McMillian.[19] Quinto also starred in the comedy short Hostage: A Love Story, written by the comedy duo HoltandSteele, for Before the Door and FunnyorDie.

Quinto's theatre experience includes roles in a variety of productions, including classics such as Beckett's "Endgame" at the Los Angeles Odyssey Theatres in 2003,[20] Shakespeare's Much Ado About Nothing at the Los Angeles Shakespeare Festival and Intelligent Design of Jenny Chow at the Old Globe Theatre. Baseline Studio Systems announced in January 2010 that Steven Spielberg may direct a biopic about George Gershwin, which is scheduled for release in 2012, with Quinto confirmed for the lead role.[21][22] From October 2010 to February 2011, Quinto played the lead role of Louis Ironson in an Off-Broadway revival of Tony Kushner's Angels in America at the Signature Theatre, New York City.[23][24] For this role, Quinto received the Theatreworld Outstanding Debut Performance award.[25] He has modeled for magazines including GQ and August.[26][27]

In 2013, Quinto played the role of Tom Wingfield in the American Repertory Theatre's production of The Glass Menagerie by Tennessee Williams.[30] He is currently in the Broadway reprisal of the production, which opened on September 23 and is planned to close on February 23, 2014.[31]

Quinto publicly came out as gay in October 2011.[32] He explained that, after the suicide of gay teenager Jamey Rodemeyer, he realized "that living a gay life without publicly acknowledging it is simply not enough to make any significant contribution to the immense work that lies ahead on the road to complete equality."[33] Prior to his coming out, Quinto had long been an active supporter of gay rights and organizations, for instance the Trevor Project.[34] In 2009, he appeared in the one-night production Standing on Ceremony: The Gay Marriage Plays, a benefit stage reading in response to the passing of Proposition 8,[35] as well as in the play The Laramie Project: 10 Years Later, about the 1998 murder of Matthew Shepard.[36] In 2010, Quinto contributed a video to the It Gets Better Project, an Internet-based campaign that aims to prevent suicide among LGBT youth.[33] In 2012 Quinto campaigned on behalf of Barack Obama, including appearing in the video Obama Pride: LGBT Americans For Obama.[37]

Since 2010, Quinto was rumored to be dating actor Jonathan Groff. In September 2012, Quinto confirmed that he and Groff were in a relationship.[38] In July 2013, it was reported that the two had broken up.[39]

Since 2013, Quinto has been publicly dating model and artist Miles McMillan. In early 2015, Quinto said that he and his partner were moving into an apartment they purchased together. Quinto also confirmed they had been together just under two years. [40][41][42]